A defiant Volodymyr Zelenskyy has announced Ukraine is formally applying for fast-track membership of the Nato military alliance, cranking up fears of a full-blown conflict between Russia and the West.

The Ukrainian leader’s move appeared to have been prompted by Russian president Valdimir Putin holding a ceremony in Moscow to proclaim four partially-occupied Ukrainian regions as annexed Russian land, following a series of sham referendums. The land-grab breaks international law.

A video showed president Zelenskyy announcing the membership bid and then signing a document flanked by his prime minister and the speaker of parliament.

“De facto, we have already proven compatibility with alliance standards,” Zelenskyy said. “We trust each other, we help each other, and we protect each other. This is the alliance.”

The announcement is likely to stoke Putin, who sees Nato as a hostile military alliance bent on encroaching in Moscow’s sphere of influence and destroying it.

In his video speech, Zelenskyy accused Russia of brazenly rewriting history and redrawing borders “using murder, blackmail, mistreatment and lies”, something he said Kyiv would not allow.

What is Nato?

The military alliance goes by the acronym of Nato.

The North Atlantic Treaty Organisation was formed in 1949 to prevent a resurgence of nationalism and militarism in Europe after two world wars, and to deter the Soviet Union’s expansion.

Its membership has swelled to 30 member nations, and over the 1990s and 2000s its enlargement stretched further east to include the former Soviet republics of Estonia, Latvia and Lithuania.

They agree to mutual defence – military action – in response to an enemy attack. The principle goes: “An attack against one ally is considered as an attack against all allies.” This is Article 5 of the Nato constitution.

Sweden and Finland, which shares a 1,300-km (810-mile) border with Russia, sought membership to Nato earlier this year following Russia’s invasion of Ukraine, but have faced a hurdle as its application needs the approval of all 30 current members, with Turkey raising objections.

Efforts to join the US-led military alliance marked a radical break in the policy of both north east European countries given their staunch military neutrality.

Because Ukraine is not a member of Nato, Western support for Zelenskyy has fallen short of putting their troops on the ground.

Nato’s enlargement was Putin’s biggest publicly-stated grievance with the West during the build-up up to war, claiming the eastward expansion and potentially sweeping up the largest other former Soviet republic breaks promises.

Nato has been adamant it will not accept limits on the nations that it admits and has always maintained an “open door policy”.

Many will fear the countries are on a collision course that could lead to a bona fide world war.

Putin vowed to protect newly-annexed regions of Ukraine by “all available means”, a nuclear-backed threat where he also railled furiously at the West, accusing the United States and its allies of seeking Russia’s destruction.

Russia has repeatedly made clear that any prospect of Ukraine joining the world’s largest military alliance is one of his red lines and it was among the justifications he has cited for his invasion — the biggest land war in Europe since the Second World War.

The immediate ramifications of the “accelerated” Nato application weren’t clear, since it requires the unanimous support of all members. The supply of Western weapons to Ukraine has, however, put it closer to the alliance’s orbit.

But it seems unlikely Nato will accept Ukraine’s application while a war is raging. Article 5 compels fellow members to actively defend it against Russia, a commitment that goes well beyond the supply of weapons.

Ukraine’s admission to Nato has been a stated goal going back to its constitution in 2002, but at the start of the war Zelenskyy accepted his country couldn’t join at present.

“For years we heard about the apparently open door, but have already also heard that we will not enter there, and these are truths and must be acknowledged,” he said at the time.